Prophet-5 problem

[IOTech]

I have a Prophet-5 rev 3.3 here with a problem I've never seen before:

The D/A converter feeds one of the oscillators with a wrong control
voltage. By adjusting the osc. frequency on the panel, it sounds as if
one ore more of the MSB's fed to the D/A are stuck for this parameter
only. (Several short repeated scales instead of one continuous.) This
can be verified by looking at the analog bus signal. 
All other 9 oscillators are ok.
Every other function on the synth also behaves normally as far as I
can tell.
The problem is periodic and it seems to be a bad contact somewhere
causing the problem. But I haven't been able to find it.
I've replaced the working ram and resoldered all IC's at the processor
board. I've tried freezing spray. With no success...
Any help appreciated.
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Ingar, Norway.